Heretofore, an automatic transmission used in a vehicle such as an automobile has included a torque converter, gears, a brake and a multi-plate clutch. The automatic transmission requires a clutch engagement for transmission and adopts a construction in which this clutch engagement is performed by oil pressure. Therefore, sealing is required for relatively rotating portions in a hydraulics circuit, a seal ring is provided in a ring groove in one of members constructing the portions, and the seal ring is designed to slidingly contact the sidewall surface of the ring groove while being thrust on an outer circumference (or inner circumference) of a partner member by the oil pressure.
In recent years, such a seal ring has gradually been changed from a conventional ring made of cast iron to a ring made of synthetic resin which allows closer contact with the partner member and has an excellent sealing property. However, since the seal ring made of synthetic resin has a good contact property, friction torque on a slidingly contacting surface with the partner member is increased. Therefore, various techniques for reducing the friction torque have been employed.
For example, as a technique for reducing the friction torque of the seal ring made of synthetic resin, a method is known, in which fluororesin such as polytetrafluoroethylene excellent in low friction torque property is used for the base material of the seal ring (refer to Japanese Patent Application Laid-Open No. H11-21408).